The invention relates to an air jet sieve device having a housing, at least one sieve for insertion in the housing, a sieve deck, a slotted nozzle beneath the sieve deck, a drive for the slotted nozzle, an air inlet to the slotted nozzle, an air outlet through the housing and out of the chamber located beneath the sieve deck and a control unit for operation of the device. The invention also relates to a method of operating the air jet sieve device.
Air jet sieve devices of this type are used for analysis sieving to determine fineness values and particle size distributions of dry materials in powder form. Analysis sieving processes are becoming increasingly automated in an effort to rule out operating errors and to achieve a high degree of measuring accuracy and reproducibility.
An air jet sieve device of this type comprises a housing upon which a sieve with flat sieve deck is placed. The sieving chamber above the sieve deck is sealed off during sieving with a cover. Underneath the sieve deck is a chamber in which a rotating slotted nozzle is located that rotates around the vertical central axis of the sieve. During a sieving process, air is blown from below through the uniformly rotating slotted nozzle against the sieve deck. The air jet purges the apertures of the sieve gauze, thus agitating the feed material lying on the sieve. The fines portion of the feed material becomes entrained in the air jet and is transported through the sieve gauze from top to bottom into the chamber underneath the sieve and from there is discharged out of the sieving machine. The coarse particles that are larger than the mesh width of the respective sieve cannot pass through the sieve and remain on the sieve gauze after sieving.
In order to determine particle size distribution, several sieving processes must be carried out with sieves of different mesh width. To this end, the sieve residue remaining on the sieve after every sieving process is subjected to further sieving processes. The sieve residue must be weighed after every sieving process to permit determination of the particle size distribution curve. As an alternative, fresh material can be weighed in for every different sieve.
In the past, air jet sieve devices were operated manually but in the last years, there has been an effort to automate analysis sieving processes in that essential process parameters such as batch weight, sieving time, air flow rate and underpressure are automatically detected and adjusted. From the prior art, the integration of load cells into the air jet sieve device to permit automatic measurement of the batch weight is generally known. A control unit for air jet sieve devices is also known where as input variables, the mesh width of the sieve, material properties of the sample and/or application area of the material can be entered which based on previously defined sieving parameters such as underpressure can be recorded and controlled and where the sieving time is preset. It is possible in this way to carry out sieving processes in accordance with internal testing specifications and to realize exactly reproducible, automated analyses. Over and above this, sieving machines can be equipped with sensors which automatically identify the mesh width of the inserted sieve and possibly also store additional information in the sieving machine or even direct in the sieve in order to increase the analysis reliability of analysis sieving machines in general against operating errors.
The measurement and regulation of the air flow rate of air jet sieve devices is known from European patent EP 0 654 308 B1. This makes it possible to keep the gas flow constant throughout the duration of the sieving process. In addition, detection of the amount of particles in the outlet gas flow and an associated analysis abort criterion derived from this is also known from this patent. Optical detection of the particle flow is proposed.
From German patent application DE 100 22 391 A1, the measurement of dust in flowing gases under the application of the triboelectrical effect is known. This is a qualitative dust measurement. The principle bases on the transfer of charges when two substances are brought together through either contact or surface friction. The difference in charge forms the basis for the triboelectric measurement. This makes it possible to carry out a qualitative monitoring of the dust concentration and a relative allocation of the particle concentration. An exact allocation of the measuring signal to the dust concentration is only possible in cases where the speed of the air flow is constant.
The optical methods already known are disadvantageous in that they require sensitive and cost-intensive measuring technology for detecting the particles in the gas flow optically, especially in the case of abrasive products. Two components are necessary for this optical method, namely a transmitter and a receiver. These are separated from the particle flow by panes of glass and the glass must be kept free from dust, which is extremely laborious. These measures call for a large overall size of the equipment. The preset of constant sieving times—even if they do differ for sieves with different mesh widths and different materials—leads to a varying degree of stress on the material, specially in the case of materials that are not wear-resistant because of different sieving periods depending on the sieve mesh width and thus to an falsification of the measuring result.
Accordingly the prior devices and methods are not desirable and improvements in these technologies are desired. Certain improvements are now provided by the present invention.